When Dynamic Lighting breaks, it's typically due to something interrupting the JavaScript that's running in the background to obscure the map to the player. Browser addon/extensions are normally culprit #1, but if you're running a very recent version of Chrome, sometimes they introduce some bugs that interfere with Roll20's processes. I am currently running Chrome version 55.0.2883.87 m (64-bit) and dynamic lighting appears to be working as expected. If you're using one of the Beta Chrome builds, you might want to see if downgrading it to a stable version to see if that fixes things. What version of Chrome are you using?